Deep sea angler fish live in such desolate environments that it is often difficult to find a mate. They've found a solution for this: The male is about one-tenth the size of the female and can't live very well without her so when he finally encounters one he latches onto her underside like a parasitical worm. He fuses to the female's flesh, feeding off her food and blood until eventually all his organs atrophy including his brain, heart and eyes, leaving only a sperm-producing sex organ to fertilize her eggs. Females can even have multiple males attached to them.

Male cuttlefish often disguise themselves as female cuttlefish to get closer to other females and mate with them. I don't understand how this works, but they're cephalopods, and cephalopods can do anything.

The female betta fish is prone to eating her own eggs so the male frequently chases her away and he cares for them until hatching.

Some species of female cichlids carry eggs in their mouths and perform oral sex on the males to fertilize them. And while we're on the subject of fish, the freshwater mangrove rivulus is the only vertebrate in the world that is hermaphroditic and it can self-fertilize so it doesn't need to mate at all. It also has the unusual ability to live in trees for large portions of its life.